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    <meta charset="UTF-8">
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    <title>找到1到100中丢失的数字</title>
    <script>
        let arr = [1, 3, 4, 5, 6, 7];
        let n = 7;
        let lost = 0,
            sum = 0;
        for (let i = 0; i < arr.length; i++) {
            sum += i;
        }
        lost = n * (n + 1) / 2 - sum;
        console.log(lost);
    </script>
</head>

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</body>

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